(a) Field of the Invention
The present invention relates to a thin film transistor and a thin film transistor array panel.
(b) Description of Related Art
A thin film transistor array panel is used for a display device such as a liquid crystal display (LCD) or an organic light emitting display (OLED), which includes a plurality of pixels independently driven by thin film transistors (TFTs).
An LCD generally includes two panels having field generating electrodes and a liquid crystal layer interposed therebetween. This LCD displays desired images by applying electric field using the electrodes to the liquid crystal layer with dielectric anisotropy and adjusting the strength of the electric field to control the amount of light passing through the panels. In this case, TFTs are used for controlling signals transmitted to the electrodes.
An OLED is a self emissive display device, which displays images by exciting an emissive organic material to emit light. The OLED includes an anode (hole injection electrode), a cathode (electron injection electrode), and an organic light emission layer interposed therebetween. When the holes and the electrons are injected into the light emission layer, they are recombined and pair annihilated with emitting light. The light emission layer further includes an electron transport layer (ETL) and a hole transport layer (HTL) as well as an electron injecting layer (EIL) and a hole injecting layer (HIL) for enhancing the light emission. Each pixel of the OLED includes two TFTs, i.e., a switching TFT and a driving TFT. The current for light emission is driven by the driving TFT and the mount of the current driven the driving TFT is controlled by the data signals from the switching TFT.
The most commonly used TFTs for an LCD adapts amorphous silicon as a semiconductor layer.
An amorphous silicon TFT has mobility of about 0.5 to 1 cm2/Vsec, which is suitable for a switching element of an LCD. However, it is not sufficient for a driving circuit of a display device.
In order to overcome such a problem, an OLED or a polysilicon TFT LCD using a polysilicon with electron mobility of 20-150 cm2/Vsec as a semiconductor layer has been developed. The relatively high electron mobility polysilicon TFT enables to implement a chip in glass technique that a display panel embeds its driving circuits.
In these display devices, the current driving capacity of the TFTs are required to be increased for the display characteristics of the pixels and for increased size and resolution of the display devices. In addition, when a driver for driving the TFTs for the pixels is integrated on the TFT array panel, the driver includes a plurality of TFTs that require higher current driving capacity than the TFTs for the pixels.
The current driving capacity of the TFTs can be increased by enlarging the channel width of the TFTs and the enlargement of the channel width also requires the increase the electrodes of the TFTs to reduce the aperture ratio of the display device since the electrodes of the TFTs are usually made of opaque metal.